輪式裝載機(jī)動(dòng)力艙熱管理系統(tǒng)數(shù)值仿真分析

1、輪式裝載機(jī)動(dòng)力艙熱管理系統(tǒng)數(shù)值仿真分析ABSTRACT：INTRODUCTION：In recent years, construction machinery manufactures have attempted persistently to deliver high engine performance and controlled climate systems. However, both attempts have encountered design hardships due to geometrical and space restrictions in the under

2、 hood compartments. 隨著工程機(jī)械行業(yè)的高速發(fā)展，人們對(duì)工程車輛安全性、舒適性、節(jié)能性和環(huán)保性的要求越來(lái)越高，如今的工程機(jī)械逐漸傾向于集成化、微型化設(shè)計(jì)，導(dǎo)致工程機(jī)械發(fā)動(dòng)機(jī)艙內(nèi)空間相對(duì)狹小以及零部件安放位置比較緊湊，艙內(nèi)布置多個(gè)熱源與散熱器,在作業(yè)過(guò)程中,經(jīng)常發(fā)生動(dòng)力艙內(nèi)散熱器模塊中,相應(yīng)各系統(tǒng)發(fā)動(dòng)機(jī)冷卻系統(tǒng)、傳動(dòng)油冷卻系統(tǒng)和液壓油冷卻系統(tǒng)溫度過(guò)高,造成相應(yīng)系統(tǒng)工作不正常,影響裝載機(jī)使用效率,同時(shí)對(duì)發(fā)動(dòng)機(jī)造成傷害。因此，研究高效可靠的裝載機(jī)動(dòng)力機(jī)艙熱管理方法勢(shì)在必行。The aforementioned facts formed components congestions

3、in vehicles underhood compartments, and therefore creating complex airflows and difficult air paths to take place within the compartments1-3. In Ref. 1 , engine cooling performance is studied by evaluating radiator heat dissipation and Top Hose Temperatures(THT) using 1D (KULI) and 3D (FLUENT, RADTH

4、ERM) simulation codes, various design modifications in underhood area are analysed through simulations. In Ref. 2, a hybrid method is presented using the principle of flow network modeling (FNM) and computational fluid dynamics (CFD), the entired flow domain in underhood is broken into various air f

5、low passages. Ref. 3 presents an analytical methodology developed to enhance passenger cars and trucks cooling and underhood thermal management process utilizing CFD technology. Ref. 4 assessed the consequence of the architectural arrangements of electrical and mechanical components on the aerotherm

6、al behavior in the underhood compartment, evaluated the qualitative impact of each individual component on the aerothermal environment of the underhood. In Ref. 5, Coupled radiation/convection simulations are performed to obtain the complete airflow and thermal map of the engine compartment using an

7、 interior-to-boundary method wherein the need for creating a water-tight surface mesh is not a pre-requisite for volume mesh generation. Ref. 6 developed a 3D CFD program which can be used by the development engineer to analyze the performance of the vehicle cooling system, and a method to predict t

8、he coolant inlet temperature of the radiator is presented. Ref. 7 expounded many individual topics which include numerical modelling of engine cooling system, under hood air flow, heat transfer at water jacket, heat transfer at radiator and coolants after-boiling phenomenon. Ref. 8 addressed the aer

9、othermal phenomena encountered in the vehicle underhood compartment by physical analysis of the heat transfer modes in complex internal flows. Ref. 9 finished with an overview of the method to link the 1-D system thermal model and the 3-D CFD model together with validation data. Ref. 10 reported a n

10、umerical investigation of spatial optimization of heat-exchanger by acting on its positioning in the vehicles cooling module, and elucidated how to act on the different parameters influencing heat-exchanger performance in order to optimize their functioning. Ref. 11 reported velocity and temperature

11、 measurements by Particle Image Velocimetry (PIV), by Laser Doppler Velocimetry (LDV) and by thermocouples, measurements are carried out for conditions simulating both the slowdown and the thermal soak phases with the fan in operation, and different fan rotational speeds, radiator water flow and und

12、erhood geometries have been experimented. However, all these attentions are focused on automobile industry. 工程機(jī)械的散熱模塊工作環(huán)境與汽車有所不同。汽車的散熱器往往前置于車頭部位，沉入動(dòng)力艙且距離進(jìn)氣格柵較近，進(jìn)氣格柵的流通面積略小于散熱器的迎風(fēng)面，往往采用迎風(fēng)面積較大、厚度較小的散熱器。而工程機(jī)械中散熱器布置特征則相反，在工作時(shí)需要保持行進(jìn)方向的準(zhǔn)確性，駕駛員需要實(shí)時(shí)觀察路面情況，因此動(dòng)力艙安裝位置不應(yīng)過(guò)高，幾何尺寸不宜過(guò)大，更不允許采用類似汽車那種大迎風(fēng)面的布置形式，動(dòng)力艙內(nèi)的散熱

13、器通常采用與冷卻風(fēng)扇居中對(duì)齊的安裝方式，迎風(fēng)面積通常略小于動(dòng)力艙截面大小，厚度較大。兩種車輛散熱器的工作狀態(tài)也有所不同，由于汽車在行駛中具有較高的迎風(fēng)速度，冷空氣受沖壓作用進(jìn)入散熱器，風(fēng)扇直徑較小。壓路機(jī)在作業(yè)中往往不具有較高車速，散熱模塊主要依靠冷卻風(fēng)扇形成的壓差，將冷空氣送入散熱器，冷卻風(fēng)扇直徑通常與散熱器的迎風(fēng)面高度或?qū)挾认喈?dāng)。Generally, there are two main sources of energy which contribute to the cooling air flow through under hood, one is the ram air and

14、another is radiator fan. As for a vehicle, the flow rate of it is determined by both fan rotational speed and vehicle proceeding speed, for the ram air, as a result of the latter, may have a considerable contribution to the magnitude of airflow blowing in. But the mass flow rate of a loader is contr

15、olled almost merely by the rotational speed of fan. Granted that a loader has the potential to move, the proceeding speed ,no more than 10m/s, is relative lower than that of a vehicle and under working condition it maybe even lower. So we just dismiss the effect of ramp air, and define a moving spee

16、d of 1m/s to consider the movement of ambient air.Compatible researches should carry on pertaining to construction machinery field where similar thermal management challenges are confronted. Whats more, 獨(dú)立散熱系統(tǒng)，quite different from the traditional heat sink system applied in loader, is seldom studied

17、 both structurally and thermally. 獨(dú)立散熱系統(tǒng)是一種應(yīng)用在裝載機(jī)上的相對(duì)新型的散熱系統(tǒng)。其特點(diǎn)是將全部的四個(gè)散熱器布置到動(dòng)力艙的后端，并在散熱器與發(fā)動(dòng)機(jī)之間設(shè)置隔熱板，thus，動(dòng)力艙被分隔為兩部分：one including風(fēng)扇、散熱器模塊 called 散熱器模塊housing，the other one including 發(fā)動(dòng)機(jī)及其他wind stymie blocks called motor housing。獨(dú)立散熱系統(tǒng)最大特點(diǎn)是風(fēng)扇由液壓馬達(dá)驅(qū)動(dòng)，其優(yōu)勢(shì)有二：其一，液壓馬達(dá)的存在避免了風(fēng)扇主軸與發(fā)動(dòng)機(jī)的直接機(jī)械連接，發(fā)動(dòng)機(jī)與風(fēng)扇的距離也就不受機(jī)械

18、連接的限制了，散熱器模塊的布置可以遠(yuǎn)離發(fā)動(dòng)機(jī)，加之隔熱板的影響，因此發(fā)動(dòng)機(jī)的熱輻射對(duì)散熱器的影響大大降低了；其二，液壓馬達(dá)的轉(zhuǎn)速可以通過(guò)溫度反饋機(jī)制調(diào)節(jié)，溫度數(shù)據(jù)來(lái)自各個(gè)散熱器溫度傳感器的監(jiān)視數(shù)據(jù)，此溫度數(shù)據(jù)作為反饋數(shù)據(jù)，若某一溫度過(guò)高，則通過(guò)此機(jī)制提高風(fēng)扇轉(zhuǎn)速，反之亦然，這就使得發(fā)動(dòng)機(jī)負(fù)載較高，燃油消耗較大。本文重點(diǎn)關(guān)注獨(dú)立散熱系統(tǒng)的設(shè)置對(duì)動(dòng)力艙結(jié)構(gòu)的影響and the sequential influence of 散熱效率。It is imperative to conduct a thermal analysis on the relatively new heat sink syst

19、em. Thus, 本論文綜合運(yùn)用一維和三維仿真方法，分析了4噸位輪式裝載機(jī)動(dòng)力艙的空氣流動(dòng)特性和散熱特性，綜合考慮了對(duì)流和輻射對(duì)散熱系統(tǒng)溫度場(chǎng)的影響，并對(duì)現(xiàn)有的動(dòng)力艙結(jié)構(gòu)形式進(jìn)行了改進(jìn)，得到了一個(gè)more optimal cooling systems 和動(dòng)力艙布置形式。2. MODELING METHODS2.1 總體研究思路（Overall research methodology）動(dòng)力艙冷卻系統(tǒng)的散熱性能由冷卻系統(tǒng)的布置形式、空氣和冷卻液的參數(shù)決定，因此，仿真模型需要能夠預(yù)測(cè)散熱器外部空氣側(cè)（air side）和內(nèi)部流體側(cè)的流場(chǎng)和溫度場(chǎng)分布。目前，大部分關(guān)于動(dòng)力艙熱管理系統(tǒng)的研究都是單

20、獨(dú)基于一維或三維CFD商業(yè)軟件的仿真研究，conventional CFD analysis for underhood thermal management is quiet involved and time consuming because of the complex geometry and flow distributions， 單一的一維仿真又由于數(shù)據(jù)的缺乏而難以保證系統(tǒng)仿真的準(zhǔn)確性。因此，本篇論文采用三維CFD仿真和一維仿真結(jié)合的方法，綜合運(yùn)用兩者的長(zhǎng)處，對(duì)輪式裝載機(jī)熱管理系統(tǒng)進(jìn)行仿真分析，總體研究思路如圖？所示。首先建立動(dòng)力艙的CAD模型，并進(jìn)行適當(dāng)?shù)暮?jiǎn)化；之后進(jìn)行網(wǎng)格的生

21、成，對(duì)重要的部位進(jìn)行網(wǎng)格的細(xì)化；前處理完成之后，將模型導(dǎo)入FLUENT求解器，設(shè)置邊界條件和初始條件，進(jìn)行流場(chǎng)求解和分析；然后，根據(jù)流場(chǎng)仿真分析得到的一些數(shù)據(jù)（比如流量、壓降），建立冷卻系統(tǒng)一維仿真模型，分析散熱器外部空氣側(cè)和內(nèi)部流體側(cè)的溫度場(chǎng)分布；最后將仿真結(jié)果與實(shí)驗(yàn)結(jié)果進(jìn)行對(duì)比，驗(yàn)證仿真結(jié)果是否合理，否則，再重新對(duì)仿真參數(shù)進(jìn)行調(diào)節(jié)，以期獲得準(zhǔn)確可信的仿真結(jié)果，并在此基礎(chǔ)上對(duì)動(dòng)力艙冷卻系統(tǒng)布置形式進(jìn)行一定的改進(jìn)，改善冷卻系統(tǒng)的散熱效果。CAD Model of UnderhoodCAD Model of UnderhoodModel SimplificationMesh Generation

22、 (HYPERMESH)CFD Modeling and Solving (FLUENT)Pressure difference and flow rate (Post-processing)1D Modeling (KULI)Characteristics Curves (Radiator, Fan, Water Circuit, Oil Circuit, Air Circuit)Air Side and Inner side Temperature DistributionComparison with different forms of layoutImprovement therma

23、l management structural styleComparison with experimental dataEndYESNORepeat with differentsetting methods2.2 整車布局（wheel loader layout）本論文以4噸位輪式裝載機(jī)為研究對(duì)象，整車外觀如圖1所示，主要由底盤（包括駕駛室）、工作裝置、動(dòng)力系統(tǒng)和散熱系統(tǒng)幾部分組成，其中動(dòng)力系統(tǒng)和散熱系統(tǒng)的總體布局如圖2所示，散熱系統(tǒng)包括發(fā)動(dòng)機(jī)冷卻系統(tǒng)（水冷卻系統(tǒng)、空氣冷卻系統(tǒng)）、液壓油冷卻系統(tǒng)和傳動(dòng)油冷卻系統(tǒng)，對(duì)應(yīng)的有水散熱器water radiator (RAD)、中冷器charge

24、 air cooler (CAC)、液壓油冷卻器hydraulic oil cooler (HOC)和傳動(dòng)油散熱器transmission oil cooler(TOC)四個(gè)散熱器。Though our main interest is focused on the underhood parts, a few of auxiliary parts are reserved because of their blockage effect. The auxiliary parts include tires, guidingroom and 工作裝置。2.3 specific featrues

25、When modeling the loader, some specific features should be 重視。For unlike traditional case, 獨(dú)立散熱系統(tǒng)的動(dòng)力艙分為兩部分: 發(fā)動(dòng)機(jī)housing、散熱器模塊housing，如圖1所示。散熱的循環(huán)系統(tǒng)分為內(nèi)循環(huán)和外循環(huán)。內(nèi)循環(huán)即散熱器內(nèi)冷卻質(zhì)的循環(huán)，冷卻質(zhì)包括發(fā)動(dòng)機(jī)冷卻水、渦輪增壓器排出的高溫空氣、液壓油及傳動(dòng)油。外循環(huán)即冷卻空氣與散熱器表面及發(fā)動(dòng)機(jī)外表面的對(duì)流換熱。若發(fā)動(dòng)機(jī)的功率和效率一定，則廢熱一定。廢熱量會(huì)在兩個(gè)housing之間分配。除通過(guò)發(fā)動(dòng)機(jī)的冷卻水內(nèi)循環(huán)外，冷卻空氣在兩者間的流動(dòng)也會(huì)影響熱量

26、的分配。這種分配會(huì)造成paradoxical effect，一方面，因散熱器housing內(nèi)的流體比較robust，the interaction有助于發(fā)動(dòng)機(jī)艙內(nèi)的熱量揮散出，另一方面通過(guò)發(fā)動(dòng)機(jī)艙由于流入散熱器艙內(nèi)的流體是高溫流體，這不利于散熱器艙內(nèi)熱量的散出。The comprehensive output of the effect is hard to decide. And 一個(gè)excellent的內(nèi)循環(huán)系統(tǒng)就是一個(gè)盡可能把發(fā)動(dòng)機(jī)的熱量帶入散熱器模塊的循環(huán)系統(tǒng)。這除了與散熱器housing的散熱效率有關(guān)外還和發(fā)動(dòng)機(jī)的循環(huán)系統(tǒng)的配置有關(guān)。我們假設(shè)內(nèi)循環(huán)系統(tǒng)excellent。Thus，a

27、ny cooling air flowing into exchanger package housing through motor housing is dispensable. The research is concentrated in the flow patterns in exchange package housing. And it seems preferable to isolate the two housing totally. But if so, a higher resistance to cooling air, and thus a lower flow

28、rate will be encountered. To evaluate this effect practically, a case comparison is conducted. 為研究散熱器艙內(nèi)的流場(chǎng)模式，必須distinguish冷卻空氣的入口和出口。在風(fēng)扇吸風(fēng)式冷卻方式情況下。散熱器艙的空氣入口有：機(jī)罩兩側(cè)開(kāi)口、機(jī)罩上部后排開(kāi)口、hiatus in chassis、 隔熱板與機(jī)罩間隙。Which can be classified as legitimate ones including機(jī)罩兩側(cè)開(kāi)口、機(jī)罩上部后排開(kāi)口, and illegitimate one， namely

29、隔熱板與機(jī)罩間隙。 For隔熱板與機(jī)罩間隙在減少流道阻力，增加流量的同時(shí)是會(huì)導(dǎo)致發(fā)動(dòng)機(jī)艙內(nèi)的高溫空氣的流入散熱器艙。我們將重點(diǎn)關(guān)注legitimate 入口的流量，及總流量在兩類入口間的分配情況。散熱器艙的空氣出口即機(jī)罩的后開(kāi)口，將重點(diǎn)研究其對(duì)散熱器艙所排出的熱空氣流動(dòng)的阻礙作用，以及其面積大小對(duì)流量大小的影響。發(fā)動(dòng)機(jī)艙的空氣入口如圖2所示，冷空氣經(jīng)由底盤及駕駛室流入發(fā)動(dòng)機(jī)艙。發(fā)動(dòng)機(jī)艙的空氣出口有兩個(gè)一個(gè)即散熱器艙的illegitimate入口，一個(gè)是機(jī)罩上部前排開(kāi)口。需要指出的是由于模型簡(jiǎn)化的原因，許多irrelevant的縫隙已被封堵，如機(jī)罩與底盤的間隙，由于此間隙較小，不會(huì)對(duì)流場(chǎng)模式及流

30、量產(chǎn)生太大影響，故在建立模型時(shí)將底盤與機(jī)罩連為一體。對(duì)于底盤底部的處理亦如此，原本在底盤內(nèi)分布有若干層的墊板及supporting board，少量空氣可由板層間的間隙流入，但是the effect is negligible，所以在散熱器艙區(qū)域的底盤底部， a single boarded is implemented. 上述的入口和出口是在風(fēng)扇吸風(fēng)冷卻的情況下demonstrated 的， 若風(fēng)扇冷卻形式改為吹風(fēng)式，則入口變出口and verse vice。3. SIMULATION3.1 網(wǎng)格生成（Grid generation）Hypermesh 11.0 is used as a t

31、ool for grid generation in the present geometry which applies the mesh-generating category of Boundary to Interior (B2I) wherein, a “water-tight” surface mesh is needed before the interior volume mesh can be generated. This means T-connections are excluded when performing every single CFD-tetramesh

32、operation. A “watertight” geometry model including the outer surface CAD data for the overall loader body structure and the underhood components should be constructed first. And a cubic box is created around the submodel as a virtual wind tunnel in which the streamwise direction represents the x-dir

33、ection and the vertical represents the z-direction, as it is shown in Fig . It defines a computational domain and, meanwhile, represents a real-world wind tunnel test environment. The location of the truck and the size of the tunnel assure that the results are not affected by the boundaries. Triangu

34、lar surface mesh elements derived from the CAD model which allows discretization of complex geometries while maintaining low cell counts are employed to generate the body mesh elements. An unstructured mesh size of approximately 17 million cells generated for the entire fluid flow domain. The finest

35、 elements are concentrated to the fan region and measure approximately 0.001 m. Figure 3a shows a cut section of the mesh in Z-0 plane of the vehicle and 3b shows surface mesh on various underhood components. Mesh quality has such a bearing on the simulation result that it is imperative to pay due a

36、ttention to control it. Whats more, unlike that of structure mesh which depends only on the sizes and shapes, mesh quality of fluid cells requires also the cell size gradient Ref. The more likely cell size gradient resembles velocity gradient, the more precise simulation result will be achieved. 3.2

37、 三維CFD流場(chǎng)模型（3D CFD Air Flow Model）Ansys Fluent 14.0 is used as a CFD simulation code for fluid flow modeling and analysis. FLUENT is a commercial software package, from Ansys software products, available for fluid dynamics simulations and quite robust in solving complicated models. The NavierStokes e

38、quations are solved through iterative procedures to satisfy mass, momentum, and energy conservation by fluent using the finite volume method. 風(fēng)洞入口定義為速度入口，入口為壓力出口。風(fēng)扇應(yīng)用mrf 模型。The fan pressure rise over the blades was obtained from experimental data and was treated as a source term in the momentum equa

39、tion. The pressure rise curve from experimentally obtained data was modeled with dimensionlessSince the fan characteristics depend on the air density in the fan blade region, the dimensionless function was correlated to the air temperature at the estimated working condition in the vehicle.散熱器被定義為多孔介

40、質(zhì)模型，The exchanger cores were modeled as rectangular fluid domains with empirical correlations for the airside pressure drop. Four porous zones were defined for HOC, CAC, RAD and TOC. The resistance coefficients were determined from the pressure drop curve provided by the component calorimeter test.

41、In the momentum equation, this pressure drop was treated as a source term. All the vents on hood are mesh screen zones, and we define for each of them an independent cell zone which means fluid zone enveloped by identified faces, namely the inner face, the outer face and the peripheral face. Despite

42、 the fact that we reserve both the outer and inner faces of hood, the wall thickness of hood is too little to define porous zones, since its thickness measures several millimeters while its height and width measures hundreds millimeters. Thus, we applied 多孔階躍模型 instead. To realize this model the inn

43、er faces of mesh screen zones are endowed the wall type of interior, while the outer faces endowed多孔階躍模型. Similarly, resistance coefficients were determined from the pressure drop curve provided by the component calorimeter test.Monitor faces definingMore than one revised is evaluated and compared i

44、n this paper. Therefore a criterion should set for comparisons and optimizations. Although the ultimate goal of this research is reduction of operating temperature on exchangers, it is both inefficiency and unnecessary to obtain the temperature data about all the studied models. As can be seen from

45、the following study, the temperature difference of a certain exchanger obtained though cooling process is bound up with its mass flow rate and the inlet temperature of the mass flow. Thus we can appraise each revised and original models based on the mass flow rate and inlet temperature of their exch

46、angers. To get mass flow rate data and speculated inlet temperature, some monitor faces should setup before calculation. Among these are the front end face of hood to monitor the mass flow rate exhausted from underhood, the lateral vents and top vents of hood to monitor the mass flow rate inhaled in

47、to under hood and the vents, formed by the gaps between chassis and insulation plate and between hood and insulation plate, to monitor the mass flow rate of relative high temperature flowing from motor housing to cooling module housing.In the computation no effort has been made to fully resolve boun

48、dary layers, instead wall functions have been used for no-slip boundaries.The computational cases presented in this paper are described in Table 1. The first case is on the present loader configuration. Thereafter, only one installation parameter is changed and evaluated at each time. And, for some

49、cases more than one installation parameter is modified relative to the basic reference case and, hence, a different reference case, namely Case REF_SP, is used in the evaluation. Each simulation has been run to convergence of mass flow through the fan. Also criteria on mass flow on mesh screens have

50、 been considered. Air Flow and Thermal AnalysisIDNameDescriptionREF1Case REFBasic reference case_2Case EXDAs Case REF but air vents on hood extended13Case PSExtended insulating plate, reaching the chassis and hood14Case P+100Insulating plate moved 100mm nearer to cooling package15Case P-100Insulatin

51、g plate moved 100mm further from cooling package16Case REF_SPA simple model with only fan and cooling model reserved-7Case F+20Fan moved 20mm nearer to cooling package68Case F+40Fan moved 40mm nearer to cooling package69Case F-20Fan moved 20mm further from cooling package610Case BLCooling pattern ch

52、anged from suction to blast 13.3 一維PTC模型（1D Power Train Cooling（PTC）Model）4. RESULTS AND DISCUSSIONFig.1and 2 show the overall velocity distribution of the loader, including both inner and outer fluid field, in the form of velocity vector where the direction of the arrows represent that of the veloc

53、ity there, the color of the arrow represent the magnitude of velocity there and the density of the arrows represents that of the nodes there. The velocity ranges of these distributions can be seen from the color-maps on the left of these figures. Threshold values of these ranges are set to obtain a

54、more illustrative figure. Regions including nodes of velocity beyond these ranges are ignored, why some regions in the exist louvers of hood abutting fan are devoid of arrows in these figures.Fig.1 is velocity distribution in perpendicular face z=0, which is the z component of central point of the f

55、an. A number of observations came out regarding the air flow behavior in the underhood region and its effects on the exchanger performance.The blockage effect of working equipment, cab, wheels and other peripheral parts can be seen conspicuously. A higher (compared to far field velocity distribution

56、) turbulent density is observed around these blockage parts. And intense vortexes due to strong turbulences have a negative effect on the flow pattern of inlet airflow. Besides, a considerable potion of kinetic energy is dissipated and altered to heat energy which aggravates local thermal condition

57、even more. The cab cause vortexes right above hood top vents which are on downstream of it. In addition, wheels and working equipment generate considerable vortexes upstream the inlet of motor housing, namely the gap between chassis and cab. Thats why blockage parts should be taken into consideratio

58、n, though they are located faraway from cooling module.Fig.1 velocity distribution on face z=0Fig.2 is velocity distribution in horizontal face y=880, which is the y component of central point of cooling fan. Blockage effect is detected from wheels on lateral inlets of hood，therefore flow pattern on

59、 the lets is worsen. The tendency of airflow inhaled into cooling module housing through lateral inlets is salient. The airflow coincides with the one inhaled from top vents and the one from gap between chase and isolation plate, thats why airflow zone between R2 exchangers. Then the blended airflow

60、 from different vents passes though two rows of exchangers, finally exhausted from exist louvers of hood. As the flow passes through the inlets the flow becomes more unstructured. For the heat exchangers are modeled as porous media, the unsteady flow becomes of a more organized character. This is du