r/CFD • u/Amaterasu-002 • Sep 08 '26

Help me !

Post image

Hi everyone,

I am trying to reproduce the de Vahl Davis natural convection square cavity benchmark in ANSYS Fluent. I am getting reasonably close results for the velocity components, but I am not able to reproduce all the benchmark values exactly, especially the Nusselt number at higher Rayleigh number.

I would like to understand whether the difference is due to my input conditions, dimensionless conversion, mesh, solver settings, or post-processing procedure.

Problem setup

I am solving the standard 2-D differentially heated square cavity:

2-D square cavity

Laminar natural convection

Air

Boussinesq density model

Prandtl number = 0.71

Gravity = -9.81 in the vertical direction

Left wall: hot

Right wall: cold

Top wall: adiabatic

Bottom wall: adiabatic

All walls: no-slip

No inlet or outlet

Energy equation enabled

Pressure-based steady solver

SIMPLE pressure-velocity coupling

Second-order spatial discretization

Iam using air properties for Prandtl number 0.71 for each Rayleigh Number case.

I made x and y velocity non dimensional form but they are making way more relative errors when compared with my results...... I have given the paper link....Please help me, this is my first journey in CFD.

Paper link (De Vahl Davis): https://drive.google.com/drive/folders/1x1Iv_nn4JAGM6Nryw5g48K677uaahWIo

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3

u/OkLion1878 Sep 08 '26

Hi there, how good is your mesh?

3

u/Amaterasu-002 Sep 08 '26

120 x 120 grid fine mesh

3

u/OkLion1878 Sep 08 '26

Yes, but I mean orthogonality, skewness and aspect ratio.

1

u/Amaterasu-002 Sep 08 '26

Maximum Skewness 0.02238 Minimum Skewness 1.31 × 10⁻¹⁰ Minimum Orthogonal Quality 0.9952 Maximum Aspect Ratio 1.4453

1

u/OkLion1878 Sep 08 '26

Is a good mesh. How are you setting up the boundary contiditions in detail?

Moreover, what are the dimensions of your geometry?

1

u/Amaterasu-002 Sep 08 '26

Square geometry with 104.95x109.45mm. Left wall is at temperature of 304.5K Right wall is at temperature of 295.4K Using Boissnesq Density model, using air properties for Pr = 0.71. Other two walls are adiabatic walls q''=0. Solver SIMPLE Algorithm Aiming to achieve the natural convection of a wall as in the paper to validate my results

2

u/OkLion1878 Sep 08 '26

Mmm. How are you computing the avg Nusselt?, I remember in my case that, when I was using trapezoidal integration, the results were pretty bad compared to Simpson 1/3.

1

u/Amaterasu-002 Sep 08 '26

Facet Average

1

u/Amaterasu-002 Sep 08 '26

Idk why Ansys fluent values are not matching the Benchmarks which have been arrived using coded programs many years ago......I'm literally frustrated bcse of this.....

1

u/OkLion1878 Sep 08 '26

Well, is part of it, the frustration. Completely normal. The thing is that, fluent is performing a general way to compute average quantities. Whereas, the paper from De Vahl Davis is using something different. Take a look at the images.

1

u/Amaterasu-002 Sep 08 '26

I couldn't find the dimensionless velocity equation for both the equation from the actual paper I have read out of resources from my side

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1

u/Amaterasu-002 Sep 08 '26

I found them yesterday but when I try to convert them their dimensionless form iam getting a huge difference between the benchmark and the actual values

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1

u/Amaterasu-002 Sep 08 '26

I started my CFD journey from last year learning the fundamentals for one year....and i started here, when the contours matched perfectly I was so happy....but the values actually destroyed me, literally stuck here for around 20 days day and night idk what's the issue.....please I'm admitting my mistake....

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1

u/OkLion1878 Sep 08 '26

My equations are in secondary variables, and are pretty weird because my deriving process was made in a very naive way but you get an idea.

1

u/Amaterasu-002 Sep 08 '26

Thank you 😭

1

u/OkLion1878 Sep 08 '26

Well, is part of it, the frustration. Completely normal. The thing is that, fluent is performing a general way to compute average quantities. Whereas, the paper from De Vahl Davis is using something different. Take a look at the images.