Turbulent and multiphase flows, owing to their ever-increasing applications, have gained a lot of attention from both academia and industry. Enhanced oil recovery, to nanofluid, to combustion can lie within the scope of turbulent and multiphase flows. Physics of Fluids hosted a special issue on this topic last year that attracted original high-quality papers on recent developments in the field of multiphase and turbulent flows applied to chemical and mechanical engineering applications. Fourteen papers have been published that cover a variety of subjects from heat transfer in transitional and turbulent flow to fully resolved complex multi-physics multi-phase flow systems.

Regarding turbulent flows, Mendez et al.1 studied the development of a laminar flow boundary layer over a concave surface and its transition to turbulence due to Gortler instability using direct numerical simulations (DNSs). They performed, for the first time, the DNS simulations of wall heat transfer effects in the receptivity, growth rate, and the development of primary and secondary instabilities up to their final breakdown to turbulence. Furthermore, the first and second momentum statistics in a fully turbulent region for such flows are reported by the authors. Doranehgard and Dehghanpour2 used a turbulence analogy to quantify diffusive and convective transport during the dissolution of CO2 in oil. Their results reveal that fingers introduced within the oil bulk phase, due to the existence of the convective flow, play an important role in diffusion coefficient calculations. Balasubramaniyan et al.3 experimentally investigated hydrodynamic instability of a lean-premixed flame. They observed that, owing to the effect of turbulent flow, the varicose mode is convectively unstable, and the sinuous mode is globally unstable as a result of Bénard–von Kármán instability. Jin et al.4 studied the effect of Stefan flow on the Nusselt number and drag coefficient for supercritical water flowing over a fixed spherical particle. Their results indicate that increasing Stefen flow intensity increases the velocity and temperature boundary layer thickness from one side and decreases Nusselt number and drag coefficient from the other side. Fang et al.5 applied an effective 3D DNS to investigate flow past bubbles using a non-body fitted gas-liquid interface tracking scheme. Using their method, they parameterically investigated the vortex structure and the evolution of the flow field. Fattahi et al.6 theoretically investigated entropy wave generation and propagation in a gas turbine combustor. They found that the unsteady chemical entropy generation is due to the existence of fluctuations in the mixture fraction. Also, they show that neglecting chemical entropy generation leads to the wrong prediction of the combustion instabilities. Farnoud et al.7 used large eddy simulation (LES) to study the particle deposition in pulsating, bi-directional nasal drug delivery. They observed that increasing the pulsation flow rate increases nasal deposition efficiency. Also, increasing drug particle size increases nasal deposition efficiency as well. Li et al.8 used linear stability analysis to theoretically study the partial vortex shedding in Taylor–Culick flow. They investigated the role of amphidromic points and revealed that these points divide the flow into two different regions: an inner region with weak perturbations and an outer region with stronger perturbation.

Concerning multiphase flows, Keepaiboon et al.9 experimentally investigated the two-phase flow boiling heat transfer characteristics of a refrigerant in a microfluidic channel at high mass fluxes. A new universal correlation is proposed for two-phase flow boiling in such systems that accounts for the important dimensionless numbers, such as the Reynolds, boiling, and Weber numbers, that potentially may provide a design basis for the micro-heat exchanger micro-electro-mechanical systems and electric vehicle battery cooling systems. Rahmat et al.,10 using the smoothed particle hydrodynamic (SPH) method, investigated the settling particle agglomeration in a dewatering process. They observed that agglomeration happens in four different structures based on the interplay between the pair potential and Reynolds number. Saghatchi et al.,11 by the means of the incompressible SPH method, investigated droplet electrohydrodynamic deformation in a superconfined domain. They found that, for some systems and confinement ratios, the influence of unbounded deformation is significant. Laurila et al.12 investigated two-phase bubbly flow in a swirl atomizer using the volume of fluid (VoF) method in OpenFoam. Their analysis revealed that the global liquid film characteristics are affected by the bubble inclusion at the nozzle inlet with relevance to atomization. Zhao et al.13 applied a computational fluid dynamics-discrete element-immersed boundary method to investigate heat transfer in compressible gas-solid flows with complex geometries. They observed that heat transfer profiles are in good agreement with the experimental data. They concluded that the proposed method would be an efficient tool to study heat transfer problems in the fluidized beds with complex geometries. Davydzenka et al.14 quantified the effect of wettability on porous media deformation. They found that wettability could affect the drag forces and control the fluid propagation patterns.

The guest editors would like to take this opportunity to thank the editorial board of Physics of Fluids, especially Professor Alan Jeffrey Giacomin (Editor-in Chief) and Dr. Matthew Kershis (Journal Manager) for their kind help and input. The first author acknowledges the support provided by Alexander von Humboldt Foundation through the project FRA-1204799-HFST-E for the experienced researcher.

1.
M.
Méndez
,
M. S.
Shadloo
, and
A.
Hadjadj
, “
Heat-transfer analysis of a transitional boundary layer over a concave surface with Görtler vortices by means of direct numerical simulations
,”
Phys. Fluids
32
(
7
),
074111
(
2020
).
2.
M. H.
Doranehgard
and
H.
Dehghanpour
, “
Quantification of convective and diffusive transport during CO2 dissolution in oil: A numerical and analytical study
,”
Phys. Fluids
32
(
8
),
085110
(
2020
).
3.
M.
Balasubramaniyan
 et al, “
Global hydrodynamic instability and blowoff dynamics of a bluff-body stabilized lean-premixed flame
,”
Phys. Fluids
33
(
3
),
034103
(
2021
).
4.
H.
Jin
 et al, “
Influence of Stefan flow on the drag coefficient and heat transfer of a spherical particle in a supercritical water cross flow
,”
Phys. Fluids
33
(
2
),
023313
(
2021
).
5.
Z.
Fang
 et al, “
Spatiotemporal evolutions of forces and vortices of flow past ellipsoidal bubbles: Direct numerical simulation based on a Cartesian grid scheme
,”
Phys. Fluids
33
(
1
),
012108
(
2021
).
6.
A.
Fattahi
,
N.
Karimi
, and
N.
Hajialigol
, “
Dynamics of entropy wave generation in a simplified model of gas turbine combustor: A theoretical investigation
,”
Phys. Fluids
32
(
10
),
106107
(
2020
).
7.
A.
Farnoud
 et al, “
Large eddy simulations of airflow and particle deposition in pulsating bi-directional nasal drug delivery
,”
Phys. Fluids
32
(
10
),
101905
(
2020
).
8.
Y.
Li
 et al, “
A theoretical study of parietal vortex shedding in Taylor–Culick flow via linear stability analysis
,”
Phys. Fluids
32
(
10
),
104101
(
2020
).
9.
C.
Keepaiboon
 et al, “
Two-phase flow boiling in a microfluidic channel at high mass flux
,”
Phys. Fluids
32
(
9
),
093309
(
2020
).
10.
A.
Rahmat
 et al, “
Modeling the agglomeration of settling particles in a dewatering process
,”
Phys. Fluids
32
(
12
),
123314
(
2020
).
11.
R.
Saghatchi
,
A.
Rahmat
, and
M.
Yildiz
, “
Electrohydrodynamics of a droplet in a highly confined domain: A numerical study
,”
Phys. Fluids
32
(
12
),
123305
(
2020
).
12.
E.
Laurila
 et al, “
Numerical study of bubbly flow in a swirl atomizer
,”
Phys. Fluids
32
(
12
),
122104
(
2020
).
13.
P.
Zhao
 et al, “
A computational fluid dynamics-discrete element-immersed boundary method for Cartesian grid simulation of heat transfer in compressible gas–solid flow with complex geometries
,”
Phys. Fluids
32
(
10
),
103306
(
2020
).
14.
T.
Davydzenka
,
S.
Fagbemi
, and
P.
Tahmasebi
, “
Coupled fine-scale modeling of the wettability effects: Deformation and fracturing
,”
Phys. Fluids
32
(
8
),
083308
(
2020
).