Summary Of Progress

Program Developments: This research program is divided into four discrete phases: Surface Cataloguing, Flat-plate Model Testing, Computational Modeling and Additional Experimental Parameters, and Model Validation in a Full-scale Test Facility. This semiannual report covers results from the Phase II Flat-plate Model Testing as well as progress made with the Phase III Computational Modeling and Additional Experimental Parameters. Progress made toward Phase IV Model Validation in a Full-scale Test Facility is addressed briefly.

Phase II:

Experimental measurements of skin friction (cf) and heat transfer (St) augmentation were made for low-speed flows over several scaled turbine roughness surface models. These tests were conducted in the AFRL Aero-thermal Research Laboratory. The models were scaled from surface measurements taken on actual, inservice land-based turbine hardware during Phase I of this effort. Model scaling factors ranged from 25 to 63, preserving the roughness height to boundary-layer momentum thickness ratio for each case. The roughness models include one pitted surface, two coated/spalled surfaces, one fuel deposit surface, and two erosion/deposit surfaces.

Measurements were made in a zero-pressure gradient turbulent boundary layers at two Reynolds numbers (Rex = 500,000 and 900,000) at low freestream turbulence levels (Tu = 1%). Measurements indicate augmentation factors ranging from 1.1-1.5 for St/Sto and from 1.3-3.0 for cf/cfo (Sto and cfo are the smooth plate values). For the range of roughness studied (average roughness height, k, less than one-third the boundary-layer thickness) the level of cf augmentation agrees well with accepted equivalent sandgrain (ks) correlations when ks is determined from a roughness shape/density parameter. This finding is not repeated with heat transfer, in which case the ks-based St correlations overpredict the measurements. Both cf and St correlations severely underpredict the effect of roughness for k+ < 70 (when ks, as determined by the roughness shape/density parameter, is small). This is the region defined by Nikuradse as being "transitionally rough" (and aerodynamically smooth for k+ < 5) where cf is a function of both Reynolds number and roughness height. A new ks correlation based on the rms surface slope angle overcomes this low k+ limitation. This parameter was first reported by Acharya et al. in 1986 and has since received only limited attention in the literature.

Comparison of data from real roughness and deterministic (ordered arrays of cones or hemispheres) roughness suggests that deterministic roughness is fundamentally different from real roughness. Specifically, ks values that correlate cf for both deterministic and real roughness are found to correlate St for deterministic simulated roughness but overpredict St for real roughness. A proposed physical explanation for this finding is advanced in the attached full report (IGTI 2002 paper submitted by Bons). These observations expose limitations in the traditional equivalent sandgrain roughness model and the common use of ordered arrays of roughness elements to simulate real roughness surfaces.

Portions of this data set will be presented at the 40th AIAA Aerospace Sciences Meeting and Exhibit in Reno (Jan 2002) by Cpt Jess Drab, one of the AFIT masters students who completed his work last year. A more comprehensive summary of findings from this phase of the effort has been submitted for presentation at the 2002 IGTI Conference in Amsterdam. A copy of this draft manuscript is included in this report. As always, great care is being exerted to maintain complete manufacturer anonymity in all published documents.

Phase III Experimental Effort:

Measurements were made at the same two Reynolds numbers (Rex = 500,000 and 900,000) with three freestream turbulence levels (Tu = 1%, 5%, and 11%). Freestream turbulence of turbine flowfields is one of the significant features that can have a synergistic effect with roughness, so freestream turbulence was selected as the first additional parameter to be studied experimentally. Exploring this and other parameters such as pressure gradient are necessary in order to establish the relevance of the laboratory findings to actual turbine operating environments. These elevated freestream turbulence levels produce augmentation ratios of 1.2 and 1.5 (St/Sto) and 1.2 and 1.3 (cf/cfo) compared to the Tu = 1% flow over the smooth reference plate. The combined effects of roughness and elevated freestream turbulence are greater than their added effects suggesting that some synergy occurs between the two mechanisms. Specifically, skin friction augmentation for combined turbulence and roughness is 20% greater than that estimated by adding their separate effects and 9% greater than compounding (multiplying) their separate effects. For heat transfer augmentation, the combined effect of turbulence and roughness is 6% higher than that estimated by compounding their separate effects at high freestream turbulence (Tu = 11%). At low turbulence (Tu = 5%), there is a negative synergy between the two augmentation mechanisms as the combined effect is 12% lower than that estimated by compounding their separate effects. These Phase III data are included in the paper that has been submitted for presentation at the 2002 IGTI Conference in Amsterdam.

Phase III Computational Modeling Effort:

The computational modeling efforts centered about developing procedures for capturing surface characteristics of real surface roughness into the discrete-element roughness model (validated originally for deterministic roughness) and extending the validation of the discrete-element roughness model to real surface roughness. Significant progress has been made in both of these efforts. Details of the successes in using real surface characteristics in the discrete-element roughness model and in extending the validation of the discrete-element roughness model to real surfaces are in the attached progress report by McClain and Hodge.

Previous progress reports have discussed the use of a layered-ellipse model to describe real surface roughness characteristics. A Mathcad program has been developed to take profilometer surface roughness data and extract input information for the discrete-element roughness model. The Mathcad program generates, at discrete elevations, the distributions of ellipses, eccentricities (major and minor axes), and orientations (parallel or perpendicular to the flow) for a given measured surface roughness. These distributions are the geometry input required for the discrete-element roughness model.

Using the surface roughness descriptions and the measurements of skin friction (cf) and heat transfer (St) augmentation made in the AFRL Aero-thermal Research Laboratory, the discrete-element model validation is being extended to real surfaces. As delineated in the attached report by McClain and Hodge, modifications to the form drag expressions, to account for non-circular cross-section roughness elements, used in the discrete-element roughness model resulted in adequate agreement with the skin friction experimental data from Phase II. Additional skin friction data over a more extended Reynolds range and for additional surface roughness configurations as well as a detailed estimate of skin friction data uncertainties are needed for more extensive validation of the skin friction capability of the discrete-element model for real surface roughness. Additional "tweaking" of the form drag correlations is underway in conjunction with the validation of the Stanton number prediction validation. The primary discrete-element modeling effort currently underway is the validation of the discrete-element roughness Stanton number (heat transfer) prediction capability for real surface roughness using the Stanton number data from Phase II.

Phase IV Model Validation in a Full-scale Test Facility:

Tragedy struck the AFRL Turbine Research Facility (TRF) this summer when a turbine rotor was over-torqued during shake-down tests, breaking the shaft and damaging the turbine casing. Repairs are underway to bring the facility back on line early next year. This will undoubtedly delay the testing planned for the final phase of this project. Dr. Rivir is working directly with the TRF team to ensure that we can get into the TRF as soon as possible after it is operational again. This will most likely be after April 2002. The plan is to continue with these tests beyond the final date of this contract (28 Feb 2002) and include the results in the final report (or as an addendum). As such, two possible methods for constructing rough-surface turbine vanes are being explored at present: casting of roughened vanes using stereolithography or sand-blasting an existing turbine vane set. The results of preliminary studies of these two options are expected to be completed by years' end so that necessary preparations can be made in the early part of 2002.

Interactions with Industry: Phase I results from this contract were well received during a presentation at the 2001 IGTI conference in New Orleans (June 2001). Discussions with numerous industry and academic representatives have continued from that time. Roughness measurements from Phase I have been shared with University of Idaho researchers who are performing AFOSR funded roughness research in the DOE INEL facility.

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