Jessica Walker

The Application of Wall Similarity Techniques to Determine Wall Shear Velocity in Smooth and Rough Wall Turbulent Boundary Layers

Publisher: ASME International

Date: 11-03-2014

DOI: 10.1115/1.4026512

Abstract: Smooth and rough wall turbulent boundary layer profiles are frequently scaled using the wall shear velocity u*, thus it is important that u* is accurately known. This paper reviews and assesses several wall similarity techniques to determine u* and compares results with data from the total stress, Preston tube, and direct force methods. The performance of each method was investigated using experimental repeatability data of smooth and rough wall turbulent boundary layer profiles at Reθ of 3330 and 4840, respectively, obtained using laser Doppler velocimetry (LDV) in a recirculating water tunnel. To validate the results, an analysis was also performed on the direct numerical simulation (DNS) data of Jimenez et al. (2010, “Turbulent Boundary Layers and Channels at Moderate Reynolds Numbers,” J. Fluid Mech., 657, pp. 335–360) at Reθ = 1968. The inner layer similarity methods of Bradshaw had low experimental uncertainty and accurately determined u* and ε for the DNS data and are the recommended wall similarity methods for turbulent boundary layer profile analysis. The outer layer similarity methods did not perform well, due to the need to simultaneously solve for three parameters: u*, ε, and Π. It is strongly recommended that the u* values determined using wall similarity techniques are independently verified using another method such as the total stress or direct force methods.

Drag force and surface roughness measurements on freshwater biofouled surfaces

Publisher: Informa UK Limited

Date: 23-04-2010

DOI: 10.1080/08927014.2010.482208

Abstract: The detrimental effect of biofilms on skin friction for near wall flows is well known. The diatom genera Gomphonema and Tabellaria dominated the biofilm mat in the freshwater open channels of the Tarraleah Hydropower Scheme in Tasmania, Australia. A multi-faceted approach was adopted to investigate the drag penalty for biofouled 1.0 m x 0.6 m test plates which incorporated species identification, drag measurement in a recirculating water tunnel and surface characterisation using close-range photogrammetry. Increases in total drag coefficient of up to 99% were measured over clean surface values for biofouled test plates incubated under flow conditions in a hydropower canal. The effective roughness of the biofouled surfaces was found to be larger than the physical roughness the additional energy dissipation was caused in part by the vibration of the biofilms in three-dimensions under flow conditions. The data indicate that there was a roughly linear relationship between the maximum peak-to-valley height of a biofilm and the total drag coefficient.

Turbulent boundary-layer structure of flows over freshwater biofilms

Publisher: Springer Science and Business Media LLC

Date: 07-11-2013

DOI: 10.1007/S00348-013-1628-X

Performance of a horizontal axis marine current turbine– A comprehensive evaluation using experimental, numerical, and theoretical approaches

Publisher: Elsevier BV

Date: 04-2018

DOI: 10.1016/J.ENERGY.2018.02.007

Impact of diatomaceous biofilms on the frictional drag of fouling-release coatings

Publisher: Informa UK Limited

Date: 26-11-2015

DOI: 10.1080/08927014.2015.1108407

Abstract: Skin-friction results are presented for fouling-release (FR) hull coatings in the unexposed, clean condition and after dynamic exposure to diatomaceous biofilms for 3 and 6 months. The experiments were conducted in a fully developed turbulent channel flow facility spanning a wide Reynolds number range. The results show that the clean FR coatings tested were hydraulically smooth over much of the Reynolds number range. Biofilms, however, resulted in an increase in skin-friction of up to 70%. The roughness functions for the biofilm-covered surfaces did not display universal behavior, but instead varied with the percentage coverage by the biofilm. The effect of the biofilm was observed to scale with its mean thickness and the square root of the percentage coverage. A new effective roughness length scale (keff) for biofilms based on these parameters is proposed. Boundary layer similarity-law scaling is used to predict the impact of these biofilms on the required shaft power for a mid-sized naval surface combatant at cruising speed. The increase in power is estimated to be between 1.5% and 10.1% depending on the biofilm thickness and percentage coverage.

Rapid Reserve Generation from a Francis Turbine for System Frequency Control

Publisher: MDPI AG

Date: 07-04-2017

DOI: 10.3390/EN10040496

The influence of surface gravity waves on marine current turbine performance

Publisher: Elsevier BV

Date: 12-2013

DOI: 10.1016/J.IJOME.2013.11.003

Light as a key driver of freshwater biofouling surface roughness in an experimental hydrocanal pipe rig

Publisher: Informa UK Limited

Date: 31-05-2016

DOI: 10.1080/08927014.2016.1184255

Abstract: Biofouling in canals and pipelines used for hydroelectric power generation decreases the flow capacity of conduits. A pipeline rig was designed consisting of test sections of varying substrata (PVC, painted steel) and light levels (transparent, frosted, opaque). Stalk-forming diatoms were abundant in both the frosted and transparent PVC pipes but negligible in the painted steel and opaque PVC pipes. Fungi were slightly more abundant in the painted steel pipe but equally present in all the other pipes while bacterial ersity was similar in all pipes. Photosynthetically functional biofouling (mainly diatoms) was able to develop in near darkness. Different biological fouling compositions generated differing friction factors. The highest friction factor was observed in the transparent pipe (densest diatom fouling), the lowest peak friction for the opaque PVC pipe (lowest fouling biomass), and with the painted steel pipe (high fouling biomass, but composed of fungal and bacterial crusts) being intermediate between the opaque and frosted PVC pipes.

Numerical assessment of a horizontal axis marine current turbine performance

Publisher: Elsevier BV

Date: 12-2017

DOI: 10.1016/J.IJOME.2017.07.009

Design and performance evaluation of a pump-as-turbine micro-hydro test facility with incorporated inlet flow control

Publisher: Elsevier BV

Date: 06-2015

DOI: 10.1016/J.RENENE.2014.12.027

Physics Based Hydraulic Turbine Model for System Dynamics Studies

Publisher: Institute of Electrical and Electronics Engineers (IEEE)

Date: 2016

DOI: 10.1109/TPWRS.2016.2574330

Succession and physiological health of freshwater microalgal fouling in a Tasmanian hydropower canal

Publisher: Informa UK Limited

Date: 17-07-2010

DOI: 10.1080/08927014.2010.506610

Abstract: Freshwater microalgal biofouling in hydropower canals in Tarraleah, Tasmania, is dominated by a single diatom species, Gomphonema tarraleahae. The microfouling community is under investigation with the aim of reducing its impact on electricity generation. Species succession was investigated using removable glass slides. Fouled slides were examined microscopically and for chlorophyll a biomass. Chl a biomass increased steeply after 8 weeks (0.09-0.87 mg m(-2)), but increased much earlier on slides surrounded by a biofouled inoculum. Succession began with low profile diatoms such as Tabellaria flocculosa, progressing to stalked diatoms such as Gomphonema spp. and Cymbella aspera. Few chlorophytes and no filamentous algae were present. Pulse litude modulated fluorometry was used to measure the physiological health of fouling on the canal wall. Maximum quantum yield (F(v)/F(m)) measurements were consistently <0.18, indicating that the fouling mat consisted of dead or dying algae. The succession and physiological health of cells in the fouling community has broad implications for mitigation techniques used.

The Effect of Freshwater Biofilms on Skin Friction Drag

Publisher: Springer Berlin Heidelberg

Date: 2009

DOI: 10.1007/978-3-540-89465-0_332

Experimental and numerical studies of blade roughness and fouling on marine current turbine performance

Publisher: Elsevier BV

Date: 06-2206

DOI: 10.1016/J.RENENE.2013.12.012

University Of Tasmania, Australian Maritime College

Location: Australia

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United States Naval Academy

Location: United States of America

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University Of Tasmania

Location: Australia

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Linkage Projects - Grant ID: LP100100700

Start Date: 06-2010

End Date: 12-2013

Amount: $160,014.00

Funder: Australian Research Council

Linkage Projects - Grant ID: LP110200244

Start Date: 01-2012

End Date: 08-2016

Amount: $148,000.00

Funder: Australian Research Council