Project

Flow Aanalysis Over Ahmed Body

RRam Sai Chigurupati4 Mar 2020

i.Ahmed body and its importance

The Ahmed body represents a simplified, ground vehicle geometry of a bluff body type.In order to check the new model working in the ansys it is used to cross verify Its shape is simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies. This model describes how to calculate the turbulent flow field around a simple car-like geometry using the Turbulent Flow, k-epsilon interface

How to run a simulation

1.Geomentry

2.Meshing

3.Setup

4.Solving

5.Results

Geomentry:

2.Meshing

Meshing is done with 100mm element size for first enclosure and varying mesh sizes for the second enclosure which is our area of intrest. The mesh is made more accurate by adding inflation layer to capture the boundary layer data. Body nd face sizing is done to make mesh more refined.

3. Setup

Here we are considering density based model(This is choosen because we are taking >0.3 mach number) steady state,Energy equation is enabled in viscous k-epsilon odel is enabled

Material is air,velocity 50m/sec,Temperature 300K

Residual Plots are created to capture the required paramenters.

Hybrid Intialization is done

Running for 500 Iterations

Results

Charts for capturing the velocity plots by cutting the plane of countours and plotting velocity

As well as we capture pressure and velocity plots.

ii.Refining the mesh around the area of interest

Case1: 35mm element size for second enclosed as mentioned in above geomentry

Case2: 40mm element size for second enclosed as mentioned in above geomentry

Case3: 45mm element size for second enclosed as mentioned in above geomentry

Refining case 1

By changing the element size of secong small closed domain to 35mm

Nodes:90811

Elements:375586

Velocity Vectors

Pressure Contour

Velocity Contour

Velocity Graphs

Velocity at 0.5m from the inlet

Refining case 2

By changing the element size of secong small closed domain to 40mm

Nodes:73068

Elements:275387

Velocity Vector

Pressure Contour

Velocity Contour

Velocity at 0.5m from the inlet

Refining case 2

By changing the element size of secong small closed domain to 45mm

Nodes:63897

Elements:225170

Velocity Vector

Velocity Graphs

ELEMENT SIZE 35MM	ELEMENT SIZE 40MM	ELEMENT SIZE 45MM
Y axis	Velocity [ m s^-1 ]	Y axis	Velocity [ m s^-1 ]	Y axis	Velocity [ m s^-1 ]
0.288	0.000	0.288	0.001	0.288	0.015
0.290	36.438	0.289652	35.778	0.289627	33.215
0.292	41.389	0.291634	40.419	0.291578	37.305
0.294	44.332	0.294012	43.221	0.293918	39.518
0.297	47.685	0.296865	45.968	0.296727	41.313
0.300	51.239	0.300301	50.181	0.300097	45.783
0.301	51.252	0.304765	50.807	0.300898	45.962
0.307	51.834	0.31911	52.946	0.30877	47.684
0.323	53.515	0.332352	54.506	0.321275	50.373
0.328	54.088	0.332367	54.506	0.339408	54.342
0.335	54.250	0.332383	54.506	0.355426	54.546
0.349	54.563	0.35566	54.743	0.36885	54.740
0.356	54.740	0.362819	54.813	0.379773	54.904
0.358	54.797	0.37443	54.811	0.393124	54.869
0.361	54.795	0.385299	54.836	0.395525	54.867
0.365	54.792	0.391024	54.860	0.397912	54.860
0.368	54.787	0.408212	54.815	0.410005	54.841
0.384	54.770	0.421312	54.744	0.420485	54.785
0.401	54.686	0.426077	54.723	0.441794	54.662
0.405	54.659	0.434257	54.675	0.449403	54.618
0.413	54.609	0.454247	54.566	0.467729	54.518
0.419	54.593	0.461393	54.533	0.476019	54.477
0.432	54.548	0.478358	54.438	0.484256	54.434
0.448	54.494	0.485799	54.397	0.502829	54.344
0.454	54.470	0.495783	54.344	0.507775	54.318
0.465	54.417	0.50261	54.302	0.533963	54.205
0.478	54.356	0.50453	54.290	0.537827	54.186
0.490	54.284	0.515861	54.228	0.539819	54.175
0.499	54.227	0.522682	54.191	0.540323	54.172
0.508	54.173	0.525912	54.174	0.541209	54.167
0.530	54.065	0.539956	54.097	0.544423	54.149
0.532	54.051	0.54407	54.076	0.573189	53.984
0.534	54.043	0.552917	54.031	0.575513	53.972
0.539	54.020	0.578441	53.900	0.579847	53.948
0.557	53.949	0.580636	53.890	0.610734	53.776
0.561	53.928	0.585284	53.865	0.616294	53.746
0.576	53.853	0.586638	53.858	0.62837	53.684
0.586	53.804	0.616605	53.712	0.649816	53.574
0.599	53.739	0.622044	53.682	0.660984	53.524
0.610	53.679	0.629661	53.645	0.666466	53.500
0.620	53.624	0.636784	53.615	0.690575	53.391
0.639	53.528	0.641268	53.595	0.694426	53.376
0.641	53.515	0.650485	53.555	0.714933	53.292
0.643	53.508	0.650662	53.554	0.715144	53.291
0.650	53.473	0.651089	53.552	0.730609	53.229
0.664	53.424	0.668409	53.479	0.731521	53.227
0.675	53.381	0.674566	53.454	0.750984	53.197
0.677	53.375	0.686345	53.405	0.762973	53.186
0.678	53.372	0.696473	53.375	0.788	53.141
0.683	53.349	0.703982	53.350	0.888	52.712
0.690	53.324	0.710904	53.329	0.988	52.520
0.702	53.276	0.724663	53.288	1.088	52.389
0.704	53.272	0.733146	53.277	1.188	52.315
0.723	53.217	0.754233	53.225	1.288	52.291
0.731	53.201	0.76376	53.216
0.741	53.186	0.788	53.159
0.763	53.156	0.888	52.710
0.788	53.119	0.988	52.510
0.888	52.766	1.088	52.378
0.988	52.572	1.188	52.304
1.088	52.439	1.288	52.279
1.188	52.364
1.288	52.340

As you can observe the circled area because of more elements the plot is getting more smoother. So by this we cana conclude that greater the mesh greater the accuracy but the only thing is it takes more time to run the simulation

Conclusion

As we can observe differrence between 45mm,40mm,35mm element sizes we can see that we can observe plots much more clearer as te element size decrease the plots or the contour looks more detailed results as well as we can see the vector plot wake region more clearer in 35mm than 40mm,45mm element size mesh . So by this we can concle that if mesh is more refined we can capture more points and the animation or contour are visible more soother and more clearer. the best example is look at the table attached above u can see that more Y points vs Velocity points are capture infor 35mm element size mesh and the points are decrease when the mesh is getting coarse and the velocity plot smoothness is decreasing

iii.Explain the variation in pressure as to why it goes negative

Set the operating pressure to 0 Pa (to cut down on rounding errors) or tied one boundary to a low pressure (50 Pa in this case), then it is entirely possible that you will have regions of negative pressure in your domain. This is because the pressure gradient enters the Navier-Stokes equations and so it is pressure differences that drive the flow. So, in regions of separated flow, the low pressure inside that region will be relative to the lowest fixed pressure in your system and may well go negative.

Of course, you\'ll get people on here saying that (absolute) pressure can\'t be negative and they\'d be right, but for an incompressible case (constant rho), you\'re dealing with gauge pressures and not absolute ones. If, however, you were to model an ideal gas, then your operating pressure might need to be fixed at, for example, atmospheric because now a negative absolute pressure would result in negative densities and so would make things difficult to say the least.

Flow Aanalysis Over Ahmed Body

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Ship work like this
in your own course.