Date of Award:

5-2015

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Mechanical and Aerospace Engineering

Committee Chair(s)

Barton L. Smith

Committee

Barton L. Smith

Committee

Robert E. Spall

Committee

Aaron Katz

Committee

Nick A. Roberts

Committee

Eric D. Held

Abstract

In this computer age, simulations are becoming common in science and engineering. One category of simulation, Computational Fluid Dynamics (CFD), begins with physical equations but adds approximations and calibrations in order to complete solutions. Translating these equations into computer languages may cause unintended errors. If simulation results are to be used for decision making, their accuracy needs to be assessed. This accuracy assessment is the theory behind the field of Verification & Validation.

Verification involves confirming the translation of physical equations to computer language was per- formed correctly. It also features methods to detect many types of code errors. Validation is quite different in that it involves the comparison of experimental data with simulation outputs. This way, errors in simulation predictions may be assessed. Validation requires highly-detailed data and description to accompany these data, and uncertainties in these data are very important. Ideally, the validation experiment measures all information required for simulation inputs. Matching inputs ensures that any possible differences in the outputs are only due to the model.

The purpose of this work is to provide highly complete validation data to assess the accuracy of CFD simulations. The physics were mixed convection. Convection is a heat transfer mode where heat is carried by a moving fluid. Mixed convection occurs where natural convection and forced convection forces are similar, such as in low-speed flows. A vertical flat plate is heated and used for a convection boundary condition. A wind tunnel that was built specifically for validation experiments was used. Air is the working fluid whose velocity is measured using a modern optical technique called Particle Image Velocimetry. Many thermocouples were used to measure the temperature of all four walls and the inlet air. Commercial heat flux sensors (HFSs) were used in the heated wall.

The two cases were steady and transient mixed convection. Steady mixed convection was studied in two orientations. First with buoyancy in the same direction of the flow and second with buoyancy in the opposite direction. The transient data were for a ramp-down flow transient. This unsteady flow was repeated many times for ensemble-averaging of the results. In both cases, uncertainty was estimated in all of the results. Additionally, the transient case was simulated with CFD, matching inputs, and a validation study was performed.

An additional study was conducted to assess the accuracy of the commercial HFSs as they were believed to be in error. The air temperature was measured very close to the wall surface. The temperature gradient is proportional to heat flux, the rate of heat energy transfer over an area. Potential errors of this method were estimated and a thorough uncertainty study was performed. Additionally, a study was performed to identify the error source of the HFSs.

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