Hugh W. Coleman

A Methodology for Determining Experimental Uncertainties in Regressions

A methodology to determine the experimental uncertainties associated with regressions is presented. When a regression model is used to represent experimental information, the uncertainty associated with the model is affected by random, systematic, and correlated systematic uncertainties associated with the experimental data. The key to the proper estimation of the uncertainty associated with a regression is a careful, comprehensive accounting of systematic and correlated systematic uncertainties. The methodology presented in this article is developed by applying uncertainty propagation techniques to the linear regression analysis equations. The effectiveness of this approach was investigated and proven using Monte Carlo simulations. The application of that methodology to the calibration of a venturi flowmeter and its subsequent use to determine flowrate in a test is demonstrated. It is shown that the previously accepted way of accounting for the contribution of discharge coefficient uncertainty to the overall flowrate uncertainty does not correctly account for all uncertainty sources, and the appropriate approach is developed, discussed, and demonstrated.

Relaxation of the Turbulent Boundary Layer After an Abrupt Change From Rough to Smooth Wall (Data Bank Contribution)

Measurements of velocity and turbulence intensity profiles and skin friction coefficient are presented for turbulent flat-plate boundary layer flow over a test surface with a rough-to-smooth step change in surface roughness. The first 0.9 m length of the test surface is roughened with 1.27 mm diameter hemispheres spaced 2 base diameters apart in a uniform staggered array, and the remaining 1.5 m length is smooth. The profiles are compared with previous data for all-rough cases under closely matched conditions in the same facility. The skin friction data are compared with previous data for both all-rough and all-smooth cases.

Experimental characterization of a thermionic-arc discharge ionization device for flowing gases

The working characteristic of a new hollow cathode, thermionic-arc dc discharge ionization device was investigated experimentally. Under typical operating conditions the device produces a steady, field-free plasma plume by ionizing a flowing gas column. Experiments were performed with argon, nitrogen, and helium, the typical discharge pressures being 0.2–0.6 Torr. The discharge exploits the inherent stability of the point to plane geometry and the ionization efficiency of the hollow cathode. Electrically, the discharge is fed from a constant–voltage power supply and maintained below the interelectrode gap breakdown threshold. Thus the discharge current is given by Ohm’s law and is a function of the applied field and local plasma parameters. This modus operandi increases the ionization efficiency by maintaining a local thermal nonequilibrium in the discharge. The degree of nonequilibrium maintained in the plume downstream the cathode was higher in nitrogen (Te/Tgas≅8–10) than in argon (Te/Tgas≅3–4), and is...

Measurement and Calculation of Fluid Dynamic Characteristics of Rough-Wall Turbulent Boundary-Layer Flows

Experimental measurements of profiles of mean velocity and distributions of boundary-layer thickness and skin friction coefficient from aerodynamically smooth, transitionally rough, and fully rough turbulent boundary-layer flows are presented for four surfaces-three rough and one smooth. The rough surfaces are composed of 1.27 mm diameter hemispheres spaced in staggered arrays 2, 4, and 10 base diameters apart, respectively, on otherwise smooth walls. The current incompressible turbulent boundary-layer rough-wall air flow data are compared with previously published results on another, similar rough surface. It is shown that fully rough mean velocity profiles collapse together when scaled as a function of momentum thickness, as was reported previously. However, this similarity cannot be used to distinguish roughness flow regimes, since a similar degree of collapse is observed in the transitionally rough regimes, since a similar degree of collapse is observed in the transitionally rough data. Observation of the new data shows that scaling on the momentum thickness alone is not sufficient to produce similar velocity profiles for flows over surfaces of different roughness character. The skin friction coefficient data versus the ratio of the momentum thickness to roughness height collapse within the data uncertainty, irrespective of roughness flow regime, with the data formorexa0» each rough surface collapsing to a different curve. Calculations made using the previously published discrete element prediction method are compared with data from the rough surfaces with well-defined roughness elements, and it is shown that the calculations are in good agreement with the data.«xa0less

The turbulent thermal boundary layer with an abrupt change from a rough to a smooth wall

Abstract The work reported here was motivated by concern over the use of smooth heat flux gages for heat transfer measurements on the otherwise rough turbine blades. Stanton number distributions and boundary layer profiles of mean temperature, mean velocity, and turbulence intensity are reported for a surface with a step change from a rough to a smooth surface. In most cases, the Stanton number immediately downstream of the change in roughness drops below the all-smooth-wall data at the same x-Reynolds number. The alignment of the smooth surface between the bases and crests of the roughness elements is shown to have only a weak effect on the Stanton number distribution. It is concluded that use of smooth heat flux gages on otherwise rough surfaces can cause large errors. It is recommended that heat transfer data collected in this manner be used with caution.

Verification and Validation in Computational Fluid Dynamics and Heat Transfer Using Experimental Uncertainty Analysis Concepts

An approach to verification and validation (V&V) using experimental uncertainty analysis concepts to quantify the result of a validation effort is discussed. This is the approach to V&V being drafted by the American Society of Mechanical Engineers (ASME) Performance Test Code Committee, PTC 61: Verification and Validation in Computational Fluid Dynamics and Heat Transfer. The charter of the committee is “Provides procedures for quantifying the accuracy of modeling and simulation in computational fluid dynamics and heat transfer.” The committee is initially focusing its efforts on drafting a standard for V&V in computational fluid dynamics and heat transfer based on the concepts and methods of experimental uncertainty analysis. This will leverage the decades of effort in the community of experimentalists that resulted in the ASME Standard PTC 19.1 “Test Uncertainty” and the ISO international standard “Guide to the Expression of Uncertainty in Measurement.”Copyright

Implementing New Uncertainty Guidelines and Standards in Turbine Testing

The use and application of uncertainty analysis in engineering has evolved considerably over the past decade. The methods formulated in the 1970’s and 1980’s that were incorporated into ANSI/ASME Standards and that were used during the AGARD-sponsored Uniform Engine Test Programme (UETP) have been superseded by more rigorous approaches. In this paper, the uncertainty methodogy used in the UETP is compared with the newer approaches that will need to be implemented in future turbine test programs.Copyright