Research

Interest.

Laminar-to-Turbulence Transition of Thermal Fluid Flows over Flat Surfaces

I began examining the laminar-turbulent transition of thermal boundary layers in my PhD studies, using a combination of DNS-based stability analysis, laser diagnostics and scaling analysis. My work revealed the resonance-triggered heat transfer enhancement of natural convection (J. Fluid Mech., Zhao et al. 2013, vol. 724). After completing my PhD, I worked on Australian Research Council-funded projects on the fundamentals of thermal boundary layers. On the projects I revealed controlled transitions (J. Fluid Mech., Zhao et al. 2017, vol. 824) of thermal boundary layers induced by selected momentum perturbations.

My research impact in this area has been recognised by the research community. An invited keynote lecture summarizing my recent years’ findings was presented at the 16th International Heat Transfer Conference (IHTC), Beijing, 2018.

PIV measurements, 2017

Developing concurrent PIV-LIF measurements using non-toxic dyes for urban microclimate modeling

Simultaneous velocity and temperature field measurement has long been a challenge. It is even more challenging to carry out such a measurement in large water tunnels where dyes have to be non-toxic (to comply with discharge requirement) and in-situ temperature-intensity calibration has to be adopted (to ensure the same spatial variation of laser intensity).

My recent work has made decisive progress that our simultaneous PIV-LIF measurement technique using non-toxic dyes and our in-house calibration setup can be adopted for urban microclimate studies where urban heat has to be modelled. A case study below showing the temperarture and velocity field obtained by a PIV-LIF measurement. Its high spatial and temporal resolutions offer new opportunties for many resarch fileds concerning buoyant and environemental fluid flows.

Simultaneous PIV-LIF measurement results, 2020

Urban Buoyant Flows in the Time of Climate Change

Urban street canyon flows play a central role in microclimate control, from street canyon to neighbourhood and city scale, which affect pollutant dispersion, thermal comfort of residents and building energy consumption for various indoor and outdoor flow conditioning systems.

How to quantify buoyancy effects on street canyon flows therefore still remains a question, in particular in cases with heterogeneous surrounding buildings. We are investigating the change in the driving factor of street canyon flows among buoyancy effect, wind conditions and canyon morphology.

Wind tunnel measurement results, 2020

Modeling cooling effects of trees

Vegetation alters the thermal and wind condition of the urban microclimate through several coupled multi-physical mechanisms, including shading, radiation trapping, evapotranspiration, and aerodynamic influence. We performed computational fluid dynamics (CFD) simulations for an idealized street canyon with linden trees . Turbulent airflow, heat and moisture transport, shortwave and longwave radiation, shading, and transpiration were fully coupled and solved in OpenFOAM. Meteorological data, including air temperature, wind speed, moisture, and shortwave radiation of the heatwave in Zurich (June 2019), were applied as boundary conditions.The simulation results reveal that the tree properties can significantly affect the cooling performance during heatwaves.

Simulation results of Linden trees showing shadeing and aerodynamic effects

Understanding Urban Heat Island Using ML

Part of my most recent research focuses on urban heat island effect in the event of heatwaves which is one of the most paramount factors responsbile for the increase of mortality in extreme weather conditions. Validated simulaitons (WRF) along with machine learning (ML) techniques are being adopted to reveal complex dependence of urban heat island on urban morphologies, anthropogenetic heat and wind dynamics.

Paris, 2019

Aligning urban cooling with global warming and heat extremes

We interpret the 30-year upward trend and spikes in urban cooling demand from the perspective of climate change, urbanization, and background climates, focusing on five representative cities: Hong Kong, Sydney, Montreal, Zurich, and London. An unequivocal, worrying upward trend in cooling demand is observed in meteorological data from 1990 to 2021, using cooling degree hours (CDH) as a city-scale metric.

Further, our quantification of the impact of the base temperature, in relation to the historical CDH, reveals that a 20% energy saving could be achieved instantly within a rather broad range of temperature and humidity by increasing the setpoint temperature by one degree, while characteristic sensational and physiological levels can be maintained at ‘acceptable’ and ‘physiological thermal neutrality’ respectively. However, the potential of reducing cooling demand can be nonlinearly and significantly lowered due to the presence of compound high relative humidity and high air temperature.

Zurich

Hong Kong

Sydney